1. Field of the Invention
The present invention relates to disease resistant plants, in particular plants resistant to organisms of the kingdom Fungi and the phylum Oomycota, the oomycetes. The invention further relates to plant genes conferring disease resistance and methods of obtaining such disease resistant plants for providing protection to Oomycota pathogens.
2. Description of Related Art
Resistance of plants to fungal and oomycete pathogens has been extensively studied, for both pathogen specific and broad resistance. In many cases resistance is specified by dominant genes for resistance. Many of these race-specific or gene-for-gene resistance genes have been identified that mediate pathogen recognition by directly or indirectly interacting with avirulence gene products or other molecules from the pathogen. This recognition leads to the activation of a wide range of plant defense responses that arrest pathogen growth.
In plant breeding there is a constant struggle to identify new sources of mostly monogenic dominant resistance genes. In cultivars with newly introduced single resistance genes, protection from disease is often rapidly broken, because pathogens evolve and adapt at a high frequency and regain the ability to successfully infect the host plant. Therefore, the availability of new sources of disease resistance is highly needed.
Alternative resistance mechanisms act for example through the modulation of the defense response in plants, such as the resistance mediated by the recessive mlo gene in barley to the powdery mildew pathogen Blumeria graminis fsp. hordei. Plants carrying mutated alleles of the wildtype MLO gene exhibit almost complete resistance coinciding with the abortion of attempted fungal penetration of the cell wall of single attacked epidermal cells. The wild type MLO gene thus acts as a negative regulator of the pathogen response. This is described in WO9804586.
Other examples are the recessive powdery mildew resistance genes, found in a screen for loss of susceptibility to Erysiphe cichoracearum. Three genes have been cloned so far, named PMR6, which encodes a pectate lyase-like protein, PMR4 which encodes a callose synthase, and PMR5 which encodes a protein of unknown function. Both mlo and pmr genes appear to specifically confer resistance to powdery mildew and not to oomycetes such as downy mildews.
Broad pathogen resistance, or systemic forms of resistance such as SAR, has been obtained by two main ways. The first is by mutation of negative regulators of plant defense and cell death, such as in the cpr, lsd and acd mutants of Arabidopsis. The second is by transgenic overexpression of inducers or regulators of plant defence, such as in NPR1 overexpressing plants.
The disadvantage of these known resistance mechanisms is that, besides pathogen resistance, these plants often show detectable additional and undesirable phenotypes, such as stunted growth or the spontaneous formation of cell death.